droplets of dilute sulfuric acid, which contribute to acid rain. Some very small particles,
such as sea salt crystals entrained into the atmosphere and formed by evaporation of
water from ocean spray, act as condensation nuclei around which raindrops form.
8.3. Energy and Mass Transfer in the Atmosphere
As discussed in Chapter 6, Section 6.2, and illustrated in Figure 6.2, the flux of
energy reaching Earth’s atmosphere is 1,340 watts/m^2. This is an enormous amount of
energy. As shown in Figure 8.3, some of the incoming energy reaches Earth’s surface,
some is absorbed in the atmosphere, warming it, and some is scattered back to space.
The energy that comes in as visible and short wavelength electromagnetic radiation (at a
maximum intensity of 500 nanometers in the visible region), must go out, which it does
as infrared radiation (with maximum intensity at about 10 micrometers (μm), primarily
between 2 μm and 40 μm). Water molecules, carbon dioxide, methane, and other minor
species in the atmosphere absorb some of the outbound infrared, which eventually is all
radiated to space. This temporary absorption of infrared radiation warms the atmosphere
— a greenhouse effect.
Some reflected from
atmosphere, clouds,
Earth’s surfaceSome absorbed
by atmosphere
Infrared outInfrared absorbedInfrared re-emittedEnergy redistributed
away from equatorSolar radiation
reaching atmosphereFigure 8.3. Some of the solar energy reaching the top of the atmosphere penetrates to Earth’s surface,
some is absorbed in the atmosphere, which warms it, and some is scattered by the atmosphere and from
Earth’s surface. Outgoing energy is in the infrared region. Some of it is temporarily absorbed by the
atmosphere before being radiated to space, causing a warming (greenhouse effect). The equatorial regions
receive the most energy, part of which is redistributed by warm air masses and latent heat in water vapor
away from the equator.
Chap. 8. Air and the Atmosphere 201